a) Field of the Invention
This invention relates to an acoustic lens system for forming an image of an object by means of ultrasonic waves and the like.
b) Description of the Prior Art
Recently, apparatus utilizing ultrasonic waves for performing observation, inspection and diagnosis of objects has been developed in relation to various ultrasonographs and ultrasonic microscopes. Each of these apparatuses is adapted to use an acoustic lens and to converge ultrasonic waves generated from a source of sound at a desired position, thereby securing an image of the surface of the object or of the inside thereof in virtue of their echoes from an object. However, most of conventional well-known acoustic lenses, which have no two-dimensional imaging function, need to move a converged point of the ultrasonic waves on the surface of the object for scanning, by moving the object, in order to obtain the image having a certain extended area of the surface of the object, and encounter the problem that the mechanical construction of the device becomes large in scale.
In contrast to this, a system has been devised which is intended to impart the two-dimensional imaging function to the acoustic lens and to bring about the image of the certain extended area without moving the object.
FIG. 1 shows an example of an ultrasonic system of this type. This system is equipped with a transducer 1 comprising a large number of minute ultrasonic elements arrayed in a lattice pattern and an acoustic lens system 2. Each of the ultrasonic elements of the transducer 1 is adapted to be excited by a pulse generator 3 for generation of ultrasonic waves and to receive the ultrasonic waves reflected from the object (the ultrasonic element serves as a transmitter and also as a receiver). The space between the transducer 1 and the object is filled with water or the like.
In the ultrasonic system, one of the ultrasonic elements first produces pulse-like ultrasonic waves, which are converged on the object by the acoustic lens system 2. The ultrasonic waves reflected from the object are converged in the reverse direction on an original ultrasonic element by the acoustic lens system 2 and converted into electrical signals through the ultrasonic element. Then, an adjacent ultrasonic element located in the same line behaves in a like manner. By the repetition of such procedure, after the scanning of one line is completed, the scanning proceeds to the next line. When all the ultrasonic elements finish such behavior, the electrical signals are secured which represent the image of an area on the object corresponding to the size of the ultrasonic transducer 1. The electrical signals are processed by a signal processing circuit 4 to display the object image on a monitor TV 5.
The acoustic lens used in the foregoing system needs to have favorable imaging performance not only at an on-axis position but also at an off-axis position. In the conventional example, however, although the idea that the ultrasonic waves are two-dimensionally imaged is disclosed, a specific structure of the acoustic lens for materializing the idea is not in any sense taught.